1. Field Of The Invention
The present invention relates to an ion microanalyzer which is so designed as to attain a high utility factor and a high analytic accuracy in the direction of the depth of a sample.
2. Description Of The Prior Art
Recently, there are increasing demands, in the field of material engineering, for the microanalysis of the microscopic domains of a solid sample and for the quantitative analysis of a very thin surface layer and many investigators have developed numerous methods to fulfill such requirements. Their researches are the basic studies to develop new materials and the ion microanalyzer has been invented in this background.
The ion microanalyzer (hereafter referred to as IMA for brevity) is an apparatus in which the secondary ions emitted from a solid sample as a result of the bombardment of the sample by the primary ion beam emitted from an ion source are analyzed by classifying the secondary ions according to their mass to electric charge ratios with a mass spectrometer.
The function of the ion microanalyzer is analogous to that of the electron probe microanalyzer (EPMA). Namely, and EPMA with its electron beam and X-ray analyzing section replaced respectively by an ion beam and a mass spectrometer can function as IMA. The IMA has the following features:
1. the treatment of the sample is easy, PA1 2. the distribution of elemental concentration in the direction of the depth of the sample can be measured, PA1 3. light elements can be analyzed, PA1 4. thin films or thin surface layers can be analyzed, PA1 5. much information can be obtained, and PA1 6. the time required to conduct measurements is short.
Of all the features mentioned above, a notable one which cannot be enjoyed by other analyzing apparatuses is the second item, that is, that the analysis of the sample in the direction of the depth thereof is possible. This analysis can be achieved by increasing the degree of sputtering by increasing the current of the primary ions emitted from the ion source. If the density of the current of the primary ions is kept constant, the speed (A/sec.) of the sputtering of the sample becomes constant, so that the distribution of the concentration of a specific element in the direction of the depth of the sample can be measured by measuring the corresponding specific current of the secondary ions.
The conventional IMA, however, is not provided with a means for focussing through lens action the secondary ion beam emitted from the surface of the sample when the primary ion beam hits against the sample and for forming the image of the sputtered portion of the sample by the secondary ions, but is provided only with a path correcting electrostatic lens which serves to turn the secondary ions emitted from the sample into a parallel beam. Therefore, most part of the secondary ions is blocked by, for example, a slit used in the mass analyzing system and only a small part of the total secondary ions emitted from the sample is used as an information signal. Consequently, the utility factor of the secondary ions is poor. Especially, in the case of the analysis of the sample in the direction of the depth thereof which utilizes the information of the secondary ions from an area of the sample smaller than that area of the sample which is bombarded by the primary ions, only the central portion of the secondary ion beam is taken out by blocking the peripheral portion of the secondary ion beam with a slit and detected. As a result of this process, the utility factor of the secondary ions becomes poorer so that the analytic accuracy is degraded due to the influences of noise etc.